It is only bad if you abuse it. Use negative margins sparingly. Realize that when you use negative margins that the other elements around it will also be effected. This means you can create some interesting effects or break your design.

What you want to avoid is using large negative margins on one element and then using large positive margins on another element to try to offset the undesired effect. Doing this can lead to strange layout behaviors in which the solution is to delete all the large negative and positive margins.